The concentration of electrons in an intrinsic semiconductor is $6 \times 10^{15} \mathrm{~m}^{-3}$. On doping with an impurity, the electron concentration increases to $4 \times 10^{22} \mathrm{~m}^{-3}$. In thermal equilibrium, the concentration of the holes in the doped semiconductor is
- A$18 \times 10^{-8} \mathrm{~m}^{-3}$
- B$1.5 \times 10^{-7} \mathrm{~m}^{-3}$
- C$9 \times 10^8 \mathrm{~m}^{-3}$
- D$\frac{2}{3} \times 10^7 \mathrm{~m}^{-3}$
✓ Correct answer: C
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